1. Field of the Invention
The present invention relates to a monolithic ceramic capacitor, and more particularly to a monolithic ceramic capacitor having a structure in which terminal members each comprising a metal plate are bonded to external electrodes.
2. Description of the Related Art
In general, monolithic ceramic capacitors have a rectangular shape, and are provided with external electrodes at the opposite ends thereof, respectively. Ordinarily, when such a monolithic ceramic capacitor is mounted onto an appropriate wiring substrate, the above-mentioned external electrodes are soldered directly to a predetermined conduction land on the wiring substrate whereby the monolithic ceramic capacitor is surface-mounted.
However, when the mounting is carried out by soldering the external electrodes directly onto the wiring substrate as described above, mechanical damage may be caused in the monolithic ceramic capacitor. That is, the capacitor body may be cracked or the external electrodes may be peeled from the capacitor body.
In many cases, such mechanical damage is caused, e.g., by a stress based on shrinkage occurring when solder for connection is solidified, or by a stress produced by a difference between the thermal expansion coefficients of the wiring substrate and the monolithic ceramic capacitor, or by a stress produced by the deflection of the wiring substrate.
These problems have been practically solved by a monolithic ceramic capacitor having external electrodes to which terminal members each comprising a metal plate are attached. In monolithic ceramic capacitors having such a structure, advantageously, a stress which causes the above-described mechanical damage can be mostly absorbed in the metal plates constituting the terminal members, which are distorted in response to the deflection. Accordingly, there is hardly any mechanical damage to the monolithic ceramic capacitors.
Ordinarily, the above-described terminal members are bonded to the external electrodes with a conductive bonding material, for example, solder, with specific faces of the terminal members being opposed to the external electrodes.
However, a monolithic ceramic capacitor having the terminal members bonded thereto as described above may encounter the following new problems.
In particular, when a monolithic ceramic capacitor containing barium titanate type ceramic as a dielectric is used in a high voltage or high frequency range, electrostriction tends to be generated, which is caused by piezoelectric phenomena in the dielectric in the capacitor body. A stress caused by such electrostriction is especially great in high capacitance monolithic ceramic capacitors.
When electrostriction occurs as described above, the displacement of the capacitor body caused by the electrostriction is considerably constrained by the terminal members which are bonded to the external electrodes, with the faces of the terminal members being opposed to the faces of the external electrodes. Therefore, the stress produced by the electrostriction can hardly escape.
As a result, the stress caused by the electrostriction is applied repeatedly and is concentrated on the bonding portions between the terminal members and the external electrodes. Fatigue breaking may occur in the bonding portions. In the worst case, cracks may be formed in the dielectric ceramic portion of the capacitor body. Even if such breaking or the like does not occur, the electrostriction can be transmitted to a wiring substrate or the like, resulting in resonance, which causes a phenomenon called xe2x80x9ccreakingxe2x80x9d in some cases.
The present invention, however, can provide a monolithic ceramic capacitor which can solve the above-described problems.
To solve the above-described technical problems, according to the present invention, there is provided a monolithic ceramic capacitor which comprises a chip capacitor body having external electrodes formed on the opposite end faces thereof, and plural internal electrodes formed in lamination so that each is electrically connected to a predetermined one of the external electrodes, and terminal members each formed of a metal plate bonded to a respective one of the external electrodes with a conductive bonding material. Each terminal member has a protuberance which projects toward the external electrode so that the bonding portion where the terminal member is bonded to the external electrode extends substantially linearly along a part of the external electrode.
According to the present invention, preferably, the direction in which the bonding portion is elongated substantially linearly is selected to be substantially parallel to the internal electrodes.
In the preferable form described above, the width of the bonding portion is preferably up to ⅔, more preferably up to {fraction (4/9)}, and most preferably up to ⅓ of the size of the end face of the capacitor body, the size being measured in the lamination direction of the internal electrodes.
Further, in the above-described preferred forms, preferably, the center in the width direction of the bonding portion is positioned in the range of ⅕ to ⅘, more preferably {fraction (2/8)} to {fraction (6/8)}, and still more preferably xe2x85x9c to ⅝ of the size of the end face from one side edge of the end face of the capacitor body in the lamination direction of the internal electrodes, the size being measured in the lamination direction of the internal electrodes.
According to the present invention, in a specific form, the protuberance is formed so as to be elongated continuously linearly. In this case, the protuberance may be formed by a bending line of a metal plate constituting the terminal member.
In another specific form of the present invention, the protuberance may be formed of plural protuberances which are distributed substantially linearly.
Further, the present invention may be applied to a monolithic ceramic capacitor provided with plural capacitor bodies. In this case, the terminal members are attached to the respective external electrodes of the plural capacitor bodies in common.
Moreover, the monolithic ceramic capacitor of the present invention may be provided with a case for accommodating the capacitor body while the terminal elements partially project outside. In the above instance, preferably, positioning pieces for positioning the capacitor body in the case are integrally formed in the terminal members.
Further, according to the present invention, there is provided a monolithic ceramic capacitor which comprises a chip capacitor body having external electrodes formed on the opposite end faces thereof, and plural internal electrodes formed in lamination so that each is electrically connected to a predetermined one of the external electrodes, respectively, and terminal members formed of a metal plate bonded to a respective one of the external electrodes, each terminal member having plural terminal elements arranged so as to be distributed at intervals in the width direction of the external electrode, the plural terminal elements being so constructed that each terminal element positioned at the ends in the arrangement direction has a lower constraining degree being defined as a degree of constraint of a stress produced by the electrostriction phenomenon in of the capacitor body, which is attributed to the bonding of the terminal elements to the external electrodes.
Further, the present invention has been devised based on the knowledge that the ratio of the displacement of a capacitor body occurring when electrostriction in an area mode is generated in a monolithic ceramic capacitor is zero at the center in the width direction of an external electrode, and becomes larger at a position more distant from the center. As described above, the present invention aims at reducing effects of a stress caused by electrostriction by dividing each terminal member into plural terminal elements, and constructing the plural terminal elements so that each terminal element positioned at an end in the arrangement direction, where a high displacement ratio is caused by the electrostriction, has a lower constraining-degree than the terminal elements positioned in the center where a low or zero displacement ratio is presented by the electrostriction, the constraining-degree being defined as a degree of constraint of a stress produced by the electrostriction phenomenon in the capacitor body, which is attributed to the bonding of the terminal elements to the external electrodes.
According to the present invention, to realize the above-described means for solving the problems, typically, in the plural terminal elements, the bonding areas of the terminal elements positioned at the ends in the width direction bonded to the external electrode are smaller than those of the terminal elements positioned in the center. In the above-described typical embodiment, in the plural terminal elements, the terminal elements positioned at the ends in the arrangement direction have a smaller size in the width direction thereof than the terminal elements positioned in the center.
Instead of this or in addition to this, the intervals between the terminal elements positioned at the ends in the arrangement direction and the next terminal elements may be longer than the intervals between the terminal elements positioned in the center and the next terminal elements.
According to the present invention, in a specific form, the plural terminal elements have the form that they are independent of each other. In another specific form, the terminal members each have plural terminal elements formed in a comb-teeth shape.
Moreover, in the monolithic ceramic capacitor of the present invention, preferably, the plural terminal elements formed in a comb-teeth shape are partially bent.
Also, the present invention may be applied to a monolithic ceramic capacitor provided with plural capacitor bodies.
The monolithic ceramic capacitor according to the present invention may be further provided with a case for accommodating a capacitor body while a part of each terminal member is exposed outside.
In the above instance, preferably, in the terminal members, a positioning piece for positioning the capacitor body in the case is formed integrally with at least one of the terminal elements.
Other features and advantages of the present invention will become apparent from the following description of the embodiment of the invention which refers to the accompanying drawings.